Spatial diversity satellite communications system with error control

Abstract

A spatial diversity satellite communications system adapted to provide its own error control. The diversity communications system utilizes transmitting station selection or burst switching in the up-link and both probabalistic and algebraic error control in the down link to increase message accuracy and to overcome precipitation attenuation. Specialized multiple level decision demodulators are employed to exploit the inherent redundancy supplied by having available multiple signals embodying identical messages. Information is fedback, once the system is operating, and is used to select the optimum transmitter site. However, all available receivers are utilized and the received messages are statistically evaluated to accurately reproduce the received message.

What is claimed is: 
     
       1. In a TDMA satellite communications system employing a plurality of earth stations separated by a substantial distance one from another, each earth station having a transmitter and a receiver, the improvement comprising: means for selecting one of said transmitters to transmit an encoded modulated burst of information to said satellite,   a plurality of demodulator means each connected to a corresponding one of said receivers for demodulating the message bursts received from the satellite according to a plurality of preselected thresholds and for producing demodulated output signals,   probabalistic decoding means connected to receive said demodulated output signals for producing decoded output signals and providing for error correction according to the probability of maximum likelihood of occurrance of said demodulated output signals, and   parity check means adapted to receive the decoded output signals from said probablistic decoding means for performing a parity check on said decoded output signals and for producing a single output signal representing the message transmitted by said selected transmitter.   
     
     
       2. The apparatus as claimed in claim 1 wherein said selecting means comprises: means for determining whether the attenuation of any transmission path is less than a preselected threshold T 1 ,   for selecting the transmitter to transmit the signal to the satellite whose path attenuation is less than T 1 ,   for determining whether the attenuation of any transmission path is greater than T 1  but less than a second preselected threshold T 2  which is greater than T 1 , and   for selecting alternate transmitters to transmit the signal to the satellite.   
     
     
       3. The apparatus of claim 2 further comprising: loopback information register means connected to said receivers for deriving path attenuation information from the frame synchronization portion of the received signals and connected to said means for selecting, said information for use in determining whether any path attenuation is less than T 3  and greater than T 1 , and   decision feedback information register means connected to said receivers for deriving path attenuation information from the order wire portion of the received signals and connected to said means for selecting, said information for use in determining whether any path attenuation is less than T 3  and greater than T 1 .   
     
     
       4. The apparatus of claim 1 further comprising synchronization means connected to receive said demodulated output signals for synchronizing the received demodulated signals one to another and producing synchronized output signals fed to said probabalistic decoding means. 
     
     
       5. The apparatus of claim 4 wherein said synchronization means includes a plurality of compression/expansion buffers to buffer said received demodulated output signals. 
     
     
       6. The apparatus of claim 1 wherein each of said plurality of demodulator means comprises six variable bias detector means forming six distinct boundary levels for producing four-bit digital words indicating the position of the received signals relative to said boundaries. 
     
     
       7. A satellite communication system comprising: a plurality of earth stations spaced apart one from another and each having a transmitter and a receiver for transmitting and receiving to and from the satellite coded burst modulated message signals;   switch means connected to receive said message signals for selectively enabling only one of said transmitters at a time;   control means connected to said receivers for actuating said switch means;   demodulator means connected to each receiver for demodulating each received message signal into an output signal composed of binary words of a first number of bits;   means connected to receive each output signal from said demodulator means for comparing the corresponding words in each signal one to another and producing output signals composed of binary words of a second number of bits wherein each bit is produced according to the probability of the maximum likelihood of occurrence of each bit; and   decoder means connected to receive said output signals composed of said binary words of said second number of bits for parity checking said words and producing a single output signal representing the message transmitted by said enabled transmitter to the satellite.   
     
     
       8. The apparatus of claim 7 wherein said control means comprises the steps of: means for determining if the signal attenuation in any transmission path to and from the satellite is less than a preselected amount T 1  ;   for selecting to be enabled the transmitter whose transmission path attenuation is less than T 1  ;   for determining if the signal attenuation of any transmission path to and from the satellite lies between the limits of T 1  and a second predetermined amount T 2  greater than T 1  ; and   for selectively enabling alternate transmitters to transmit to said satellite if the signal attenuation falls between T 1  and T 2 .   
     
     
       9. The apparatus of claim 8 further comprising burst synchronization means connected to receive said demodulation output signals composed of binary words for synchronizing with each other the corresponding words in each output signal and producing synchronized output signals therefrom. 
     
     
       10. The apparatus of claim 9 wherein said demodulator means are comprised of a plurality of variable bias threshold detectors having said bias threshold levels set to form a plurality of predetermined boundary regions defined by four bit binary words. 
     
     
       11. The apparatus of claim 10 further comprising: loopback register means connected to said receiver for producing first path attenuation level output signals connected to said control means, said output signals being derived from the burst synchronization information of the received signals; and   feedback register means connected to said receivers for producing second path attenuation output signals also being connected to said control means, said second path attenuation signals being derived from the order wire segment of the received signals.   
     
     
       12. In a TDMA satellite communications system employing a plurality of earth stations each containing an transmitter and receiver, the improvement comprising: switch means for connecting only one transmitter to an encoded modulated burst of information for transmitting said information via said satellite to said earth stations;   control means connected to the outputs of said receivers for actuating said switch means in accordance with a predetermined algorithm;   a plurality of four phase demodulator means one connected to the output of each receiver for demodulating each received signal into a demodulated signal comprised of binary words;   synchronizing means connected to receive each demodulated signal for synchronizing the corresponding words in each demodulated signal to each other and producing parallel synchronized output signals therefrom;   decoder means connected to receive said parallel synchronized output signals for detecting the corresponding bits in each binary word and producing a output signal composed of binary words wherein each bit value represents the majority of the bits detected; and   means for performing parity check on said output signal from said decoder means and producing an output signal representing said encoded modulated burst of information.